Extreme-Ultraviolet Excited Scintillation of Methylammonium Lead Bromide Perovskites

Inorganic–Organic lead halide materials have been recognized as potential high-energy X-ray detectors because of their high quantum efficiencies and radiation hardness. Surprisingly little is known about whether the same is true for extreme-ultraviolet (XUV) radiation, despite applications in nuclear fusion research and astrophysics. We used a table-top high-harmonic generation setup in the XUV range between 20 and 45 eV to photoexcite methylammonium lead bromide (MAPbBr3) and measure its scintillation properties. The strong absorbance combined with multiple carriers being excited per photon yield a very high carrier density at the surface, triggering photobleaching reactions that rapidly reduce the emission intensity. Concurrent to and in spite of this photobleaching, a recovery of the emission intensity as a function of dose was observed. X-ray photoelectron spectroscopy and X-ray diffraction measurements of XUV-exposed and unexposed areas show that this recovery is caused by XUV-induced oxidation of MAPbBr3, which removes trap states that normally quench emission, thus counteracting the rapid photobleaching caused by the extremely high carrier densities. Furthermore, it was found that preoxidizing the sample with ozone was able to prolong and improve this intensity recovery, highlighting the impact of surface passivation on the scintillation properties of perovskite materials in the XUV range.


Intermittent Exposure
: Number of counts measured as a function of time in case of XUV exposure of methylammonium lead bromide with intermittent opening and closing of the shutter to block the XUV light. The graph indicates that no intermediate recovery occurs when the shutter is closed. The PL intensity continues as if the shutter never closed. Figure S2: Using an attenuation length of 75 nm (99%) absorption and a density of 3.5 g/cm 3 , 1 the dose axis can be converted to an approximate absorbed irradiation dose in Gray y (J/kg)to allow a tentative comparison with other reported scintillator photobleaching curves. It is non-trivial to compare the photobleaching rate with published bleaching rates of common scintillators, such as anthracene or plastic scintillators, due to sharp contrasts in attenuation length across the XUV spectrum used as bleaching source in our experiments, as can be seen in Fig. 1A of the main text, when compared to hard X-rays. Nevertheless, when the dose axis is converted the photobleaching performance is comparable to several plastic scintillators. 2 Figure S3: A) Full survey of Fig.3A in the main text with all visible peaks labeled B) N 1s peaks in exposed and unexposed area of sample overlaid C) N 1s Fitted D) Br 3d in exposed and unexposed area of sample overlaid E) Br 3d peak fitted S3 Figure  and k = 0 for all wavelengths. As the excitation occurs under normal incidence, the power reflection coefficient R on the boundary between vacuum (medium 1) and perovskite (medium

S2
This yields power reflectances of R 260 = 0.16, R 330 = 0.19 and R 400 = 0.17, respectively at the vacuum/perovskite boundary. We are furthermore assuming a homogeneous, Gaussian beam profile and neglecting further losses due to scattering. Due to our neglecting of scattering, the actual carrier density will be lower. Amplified spontaneous emission is clearly visible in the case of 330 nm and 400 nm excitation, so a carrier concentration above 10 19 cm −3 is expected. 5 The excitation pulses, originating from the optical parametric amplifier, have durations between 60 and 80 femtoseconds as measured with frequency resolved optical gating. We thus assume that the carrier distribution appears in an instant in the material and returns to equilibrium through radiative and non-radiative processes on a much longer time scale.
The recorded single pulse fluence in the case of 330 nm excitation was 17 mJ/cm 2 which is contained within the 150 µm full-width at half maximum (FWHM) of the Gaussian beam profile (which contains 76% of total fluence). Furthermore, it is assumed that 330 nm photons cannot excite more than one carrier per photon and that leftover energy is lost through nonradiative decay. The pulse contains 2.8 * 10 16 phot/cm 2 , calculated by converting the photon energy and fluence to electronvolts. Taking into account that 19% of these photons are reflected and not taking into account any additional scattering losses, we take the average sample thickness to find a maximum carrier concentration of 9.8 ± 0.1 * 10 20 cm −3 across the sample thickness.
For an excitation fluence of 8 mJ/cm 2 /pulse in the case of 400 nm. Using the same method as before, one finds 1.7 * 10 16 phot/cm 2 and carrier density of approximately 6.7 ± 0.2 * 10 20 cm −3 . In the case of 260 nm excitation, the excitation fluence was 50 µJ/cm 2 /pulse. This yields a much lower 6.5 * 10 13 phot/cm 2 and consequently a lower estimated carrier concentration of 3.0 ± 0.3 * 10 18 cm −3 .

Pulsed XUV excitation
In the case of XUV excitation, one can use the same method, but one has to take into account the broadband energy range (20 − 45 eV) of the XUV source and the large change in absorbance across that same range (blue curve in Fig.1a). The estimated single pulse fluence across the XUV range is approximately 26 µJ/cm 2 and the distribution of photons across photon energies ρ(eV ) corresponds to the red curve in Fig.1A. The XUV absorbance of MAPbBr 3 was calculated using atomic scattering factors 6 and as such can be converted to an energy dependent [nm −1 ] absorbance (α(eV )).
One then takes the previously calculated photon distribution and propagates it through the thin film with the theoretical absorbances according to Equ.S5 to get a carrier distribution n(eV ): This yields the distribution in Fig.S6, depicting two possible extremes. One in which one photon creates one carrier (blue curve) and one in which the photon energy is converted in the maximum possible amount of discrete carriers, rounded down (red curve). This unknown conversion factor of photons to carriers is the reason for the large range between which the carrier density can lie. Figure S6: A) Propagating XUV light through nm's of MAPbBr 3 as a function of photon energy. Colorbar indicates powers of ten. B) Number of carriers as a function of depth over the entire spotsize, varying between a low estimate of one carrier/photon (blue curve) to the maximum possible where all photon energy is converted inelastically into carriers (red curve) with an energy of 2.53 eV (exciton energy). The sum of carriers between two depth boundaries can be converted to a carrier density estimate by dividing by the spotsize.